The applicant proposes to compare the presence of JC virus DNA, detected by PCR, in six patient groups. These groups include patients with AIDS-associated PML, patients with PML without AIDS, HIV-positive patients without evidence of PML, patients with inflammatory disorders of the nervous system without HIV-related disease or PML, immunosuppressed patients without PML or HIV-related disease, and other neurologic controls. The applicant anticipates that these experiments will serve the broad, long-term objective of determining whether the presence of JC viral DNA, as detected by PCR in the cerebrospinal fluid (CSF) of patients, can be used as a non-invasive marker for the diagnosis of PML.